This invention relates generally to devices for degaussing magnetic materials and more specifically to a circuit for magnetically biasing and fast degaussing of a magnetic device.
This invention was the result of a contract with the United States Department of Energy.
Various devices are known which utilize a magnetostrictive transducer to excite various vibratory motions in an acoustic conductor or drive member of the device. One example, is a device for measuring liquid level in varying temperature and pressure environments which is the subject of a U.S. patent application Ser. No. 314,915(79) now abandoned, the disclosure of which is incorporated herein by reference thereto. In this device the liquid level is measured by immersing an elongated conductor element in the liquid and propagating ultrasonic acoustic wave energy in both longitudinal and torsional propagation modes by means of alternately pulsing a magnetostrictive transducer forming a portion of the conductor element.
In order to excite these alternate modes in a single transducer, it has been the practice to magnetically bias, or polarize, the magnetostrictive material of the transducer by either passing a small d.c. current through the material during pulsing or permanently polarizing the material by passing a large d.c. current (100 amps) through the material prior to assembly of the device. This approach works quite well if only a single mode of excitation is used. To excite a torsional wave in the rod, a coil wound about the polarized magnetostrictive rod is pulsed to generate a magnetic field which superimposes with the circumferential magnetic field in such a manner to generate the torsional wave. However, to excite a longitudinal wave in the rod, this circumferential field must be cancelled, since a longitudinal wave is produced by generating a magnetic field parallel to the rod. To accomplish this in the above-referenced device, a permanent magnet is placed near a second coil wound about the polarized rod and oriented to cancel the circumferential field, and to induce a longitudinal field.
A limitation on ultrasonic transducer systems of this type or others which alternate between different propagating modes is that any remaining magnetism from the previous magnetic field produced by the drive pulses can cause production of unwanted pulses and can also weaken the desired pulses should magnetic saturation be reached.
The solution to the problem is to demagnetize the magnetostrictive material thoroughly between alternating biasing arrangements. In a situation which requires rapid switching between various modes, as in the above-referenced system, demagnetization must be done quickly and efficiently. Thus, there is a need for a device to selectively magnetically bias a magnetic material and subsequently quickly degauss the material between alternate modes of magnetostriction.